Since it's for a fictional book, you can just make something up. If you're hpping for some recipe of exotic of grotesque ingredients in a potion, that will most likely not happen, unfortunately, as most of the "bat wings" and "eye of newt" stuff from tales was either fiction, or other things in some way codified to hide real ingredients lists (some,for examply, saying 'bat wings' wrre really holly leaves).
Not to mention many of the experienced here will tell you that not only are potions and spells *not* magic -- just tools to get a magical result, but also physical transformation through magic is not possible.
If you're looking for creative resource, however, the spells section on the site will have many possibilities for your basis, including many fantasy spells for that flair of drama.
Or you could consider things such as the way certain unripe or overripe fruits will cause thd mouth to feel like it is being drawn closed (persimmons, i think, will cause this sensation), then O'm sure tou can come up with allnsorts of creative ways to dstort a story character's features.